Synthesis gas containing hydrogen and carbon monoxide is produced for a variety of industrial applications, for example, the production of hydrogen, chemicals and synthetic fuel production. Conventionally, the synthesis gas is produced in a fired reformer in which natural gas and steam is reformed in nickel catalyst containing reformer tubes at high temperatures (e.g. 900° C. to 1000° C.) and moderate pressures (e.g 16 to 20 bar) to produce the synthesis gas. The endothermic heating requirements for steam methane reforming reactions occurring within the reformer tubes are provided by burners firing into the furnace that are fueled by part of the natural gas. In order to increase the hydrogen content of the synthesis gas produced by the steam methane reforming (SMR) process, the synthesis gas can be subjected to water-gas shift reactions to react residual steam in the synthesis gas with the carbon monoxide.
A well established alternative to steam methane reforming is the partial oxidation process (POx) whereby a limited amount of oxygen is allowed to burn with the natural gas feed creating steam and carbon dioxide at high temperatures and the high temperature steam and carbon dioxide are subjected to subsequent reforming reactions. A key shortcoming of both the SMR and POx processes is the significant amount of carbon emitted to the atmosphere as carbon dioxide gas in the low-pressure flue gas. In addition, producing synthesis gas by conventional SMR or POx processes are recognized to be a relatively expensive processes.
An attractive alternative process for producing synthesis gas is an oxygen-fired autothermal reformer (ATR) process that uses oxygen to partially oxidize natural gas internally in a reactor which retains nearly all the carbon in the high pressure synthesis gas, thus facilitating removal of carbon dioxide for carbon capture. However, the ATR process requires a separate air separation unit (ASU) to produce high purity, high-pressure oxygen, which adds complexity as well as capital and operating cost to the overall process.
As can be appreciated, the conventional methods of producing a synthesis gas such as SMR, POx or ATR systems are expensive and require complex installations. In order to overcome the complexity and expense of such installations it has been proposed to generate the synthesis gas within reactors that utilize an oxygen transport membrane to supply oxygen and thereby generate the heat necessary to support endothermic heating requirements of the steam methane reforming reactions. A typical oxygen transport membrane has a dense layer that, while being impervious to air or other oxygen containing gas, will transport oxygen ions when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the membrane.
Examples of oxygen transport membrane based reforming reactors used in the production of synthesis gas can be found in U.S. Pat. Nos. 6,048,472; 6,110,979; 6,114,400; 6,296,686; 7,261,751; 8,262,755; and 8,419,827. The problem with all of these oxygen transport membrane based systems is that because such oxygen transport membranes need to operate at high temperatures of around 900° C. to 1100° C., preheating of the hydrocarbon feed to similarly high temperatures is often required. Where hydrocarbons such as methane and higher order hydrocarbons are subjected to such high temperatures, excessive carbon formation will occur in the feed stream, especially at high pressures and low steam to carbon ratios. The carbon formation problems are particularly severe in the above-identified prior art oxygen transport membrane based systems. A different approach to using an oxygen transport membrane based reforming reactor in the production of synthesis gas is disclosed in U.S. Pat. No. 8,349,214 and United States Patent Application Serial No. 2013/0009102 both of which disclose a reactively driven oxygen transport membrane based reforming system that uses hydrogen and carbon monoxide as part of the reactant gas feed which address many of the highlighted problems with the earlier oxygen transport membrane systems. Other problems that arise with the prior art oxygen transport membrane based reforming systems are the cost and complexity of the oxygen transport membrane modules and the lower than desired thermal coupling, durability, reliability and operating availability of such oxygen transport membrane based reforming systems. These problems are the primary reasons that oxygen transport membranes based reforming systems have not been successfully commercialized. Recent advances in oxygen transport membrane materials have addressed problems associated with oxygen flux, membrane degradation and creep life, but there is much work left to be done to achieve commercially viable oxygen transport membrane based reforming systems from a cost standpoint as well as from an operating reliability and availability standpoint.
Process designs that utilize thermally coupled separate oxygen transport membrane and catalytic reforming reactors have their own set of challenges. For example, oxygen transport membranes may be configured to perform several tasks such as separation of oxygen from air, reaction of permeated oxygen with a reactant stream to produce a water vapor containing reactant stream required to support endothermic reactions in the catalytic reforming reactor and transferring heat to drive the endothermic reactions in the catalytic reforming reactor to achieve desired production of synthesis gas. Heat to support endothermic reactions within catalytic reactors is mostly provided by radiant heat transfer of the heat released from combustion of permeated oxygen in the oxygen transport membrane reactor. At elevated temperatures the oxygen transport membranes are subjected to considerable mechanical stresses both during normal steady-state operation and transient operations such as start-up, shutdown, as well as, upset conditions, particularly at detrimental levels when temperatures or rate of temperature change may be outside acceptable ranges. Thus, inefficient transfer of exothermic heat released in the oxygen transport membrane reactors to the catalytic reforming reactors will lead to less efficient operation, higher capital cost and more complex system.
The need, therefore, continues to exist for a synthesis gas generation system or other oxygen transport membrane based reactor that has a high degree of thermal efficiency. The present invention addresses the aforementioned problems by providing a method and system for air temperature control in an oxygen transport membrane based reactor to maintain generally consistent surface temperatures of the oxygen transport membrane elements and associated reactors by introducing a specific quantity of cooling air or trim air in between stages in a multistage oxygen transport membrane based reactor.